Mode lock techniques and Q-switch techniques are available for generating short pulse lasers. A mode lock technique is one in which a vertical mode being determined by an interval between oscillators is locked by modulating it with an electrooptic modulation device, for instance. Mode lock techniques are mainly applied to fiber lasers; and it is likely that pulse lasers with highly repetitive frequencies, whose pulse-time width is short, are obtainable from mode-locked fiber laser apparatuses. However, since the pulse energy of the obtainable pulse lasers is small, it is necessary to amplitude the pulse lasers in order to make them applicable to spectroscopic measurements, material processing, wavelength conversions, and so on.
On the other hand, a Q-switch technique is one in which, while a gain medium is being excited, an oscillation is suppressed by making the loss of oscillator larger so in order to lower the Q-value, and then the oscillator is caused to oscillate by making the loss smaller suddenly in order to raise the Q-value. Q-switch techniques are mainly applied to solid lasers; and it is likely that pulse lasers with lowly repetitive frequencies, whose pulse energy is large, are obtainable from Q-switch-type solid laser apparatuses.
For the Q-switch-type solid laser apparatuses, the following are available: active Q-switch laser apparatuses in which the Q-value is changed with active devices like electrooptic modulation devices; and passive Q-switch laser apparatuses in which the Q-value is changed with passive devices like saturable absorbers.
In the active Q-switch laser apparatuses, it is not possible to make the time width of pulse shorter because the active device is so large that it is not possible to make an interval between oscillators shorter. Moreover, the active Q-switch laser apparatuses have also had such a problem that they require a high voltage in order to drive the active device.
Since it is possible for the passive Q-switch laser devices to cancel the problems of the aforementioned active Q-switch laser apparatuses, the research and development have been recently carried out extensively.
John J. Zayhowski developed a passive Q-switch-type solid laser apparatus in which Nd3+:YAG (i.e., a gain medium) was diffusion joined to Cr4+:YAG (i.e., a saturable absorber), and in which they were sandwiched with oscillator mirrors (see Non-patent Literature No. 1, for instance).
In this apparatus of John J. Zayhowski, the peak power is enhanced by making the oscillator length shorter, and thereby the following are achieved: 218-ps pulse-time width; and 4-μJ pulse energy (or 18-kW peak power) at 70-kHz maximum repetitive frequency.
Yingxin Bai et al. developed a passive Q-switch-type solid laser apparatus in which Nd3+: YVO4 (i.e., again medium) and Cr4+:YAG (i.e., a saturable absorber) were used (see Non-patent Literature No. 2, for instance).
In this apparatus of Yingxin Bai et al., the following are achieved by making the gain medium's beam diameter larger than the saturable absorber's effective beam diameter: 28-ns pulse-time width; and 20-μJ pulse energy (or 0.7-kW peak power). Note that, since a laser diode (or LD) exclusively for quasi CW is used for the excitation means in this case, the repetitive frequency is limited to from a few dozens of Hz and up to 100 Hz approximately at a maximum.
H. Sakai et al. developed a passive Q-switch-type solid laser apparatus in which Nd3+:YAG (i.e., again medium) and Cr4+:YAG (i.e., a saturable absorber) were used (see Non-patent Literature No. 3, for instance).
In the apparatus of H. Sakai et al., the oscillator length is made shorter by turning the gain medium into a microchip so that the peak power is enhanced, and thereby the following are achieved: 580-ps pulse-time width; and 0.69-mJ pulse energy (or 1.2-MW peak power). Note that, in this case, the repetitive frequency is suppressed down to 100 Hz at a maximum in order to reduce thermal problems.